Sunlight inactivation of fecal indicator bacteria and bacteriophages from waste stabilization pond effluent in fresh and saline waters

Abstract

Sunlight inactivation in fresh (river) water of fecal coliforms, enterococci, Escherichia coli, somatic coliphages, and F-RNA phages from waste stabilization pond (WSP) effluent was compared. Ten experiments were conducted outdoors in 300-liter chambers, held at 14C (mean river water temperature). Sunlight inactivation (k(S)) rates, as a function of cumulative global solar radiation (insolation), were all more than 10 times higher than the corresponding dark inactivation (k(D)) rates in enclosed (control) chambers. The overall k(S) ranking (from greatest to least inactivation) was as follows: enterococci > fecal coliforms greater-than-or-equal E. coli > somatic coliphages > F-RNA phages. In winter, fecal coliform and enterococci inactivation rates were similar but, in summer, enterococci were inactivated far more rapidly. In four experiments that included freshwater-raw sewage mixtures, enterococci survived longer than fecal coliforms (a pattern opposite to that observed with the WSP effluent), but there was little difference in phage inactivation between effluents. In two experiments which included simulated estuarine water and seawater, sunlight inactivation of all of the indicators increased with increasing salinity. Inactivation rates in freshwater, as seen under different optical filters, decreased with the increase in the spectral cutoff (50% light transmission) wavelength. The enterococci and F-RNA phages were inactivated by a wide range of wavelengths, suggesting photooxidative damage. Inactivation of fecal coliforms and somatic coliphages was mainly by shorter (UV-B) wavelengths, a result consistent with photobiological damage. Fecal coliform repair mechanisms appear to be activated in WSPs, and the surviving cells exhibit greater sunlight resistance in natural waters than those from raw sewage. In contrast, enterococci appear to suffer photooxidative damage in WSPs, rendering them susceptible to further photooxidative damage after discharge. This suggests that they are unsuitable as indicators of WSP effluent discharges to natural waters. Although somatic coliphages are more sunlight resistant than the other indicators in seawater, F-RNA phages are the most resistant in freshwater, where they may thus better represent enteric virus survival.

FIG. 1.

Variability associated with sample collection, transport, and assay. Each box plot indicates the counts obtained from 10 samples, collected over 6 min from each chamber, after 3 h of sunlight or dark inactivation. The cross-pieces of each box plot represent (from top to bottom) maximum, upper-quartile, median, lower-quartile, and minimum values. An outlier (○) is defined as a point whose value is either above the upper quartile or below the lower quartile by 1.5 times the interquartile distance.

FIG. 2.

Inactivation in river water, as a function of insolation, of fecal coliforms, enterococci, somatic coliphages, and F-RNA phages from WSP effluent. The data are from all summer (○) and winter (•) survival experiments. The bacterial results are the linear portions of the inactivation curves (i.e., the shoulder points have been removed). The enterococci data are divided into winter and summer curves, with separate k_S values. Combined k_S values are presented for the other indicators.

FIG. 3.

Sunlight inactivation in river water (○), simulated estuarine water (⊙), and seawater (•) of fecal coliforms, enterococci, somatic coliphages, and F-RNA phages from WSP effluent and raw sewage. For clarity, the phage data are presented as regression lines. Note the differences in the y-axis scales.

FIG. 4.

Results of winter (A) and summer (B) 2-day experiments. Inactivation in river water of fecal coliforms (○), enterococci (•), somatic coliphages (⋄), and F-RNA phages (♦) from WSP effluent, as a function of insolation and time. During daylight hours, the insolation scale is linear and the time scale is nonlinear. During the overnight period, the insolation scale is nonlinear, the time scale is linear, and the symbol shades are inverted.

FIG. 5.

Inactivation in river water of fecal coliforms, enterococci, F-RNA phages, and somatic coliphages from WSP effluent in the dark (•), under full sun (○), and under 556-nm (orange) (▿), 396-nm (polycarbonate, □), 342-nm (acrylic, ▵), and 318-nm (polyester, ⋄) optical filters. For clarity, the phage data and most of the fecal coliform data are presented as regression lines. Note the difference in y-axis scales.